ΔE 26.2, loose: different enough that you should pick on purpose. Forest Green is a little lighter, much more saturated, cooler than Dark Green Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Forest Green is a little lighter, much more saturated, cooler than Dark Green Brown.
The same difference applies the other way round: Dark Green Brown is a little darker, much more muted, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Dark Green Brown on the colour wheel See Forest Green on the colour wheel
All matches: Dark Green Brown equivalents · Forest Green equivalents